Conventionally, when bonding semiconductor power devices such as IGBT (Insulated Gate Bipolar Transistor) and MOS-FET (Field Effect Transistor) on to metal lead frames, lead-free solder has been widely used; however, in recent years, the toxicity of lead is being considered problematic. Further, in recent years, due to the growing requirements for higher-density energy control, studies on power devices that utilize wide-gap semiconductors such as SiC and GaN, which show resistance at junction temperatures of 200° C. or higher, are moving along. However, because the eutectic melting point of lead solder is low, the lack of heat resistance at the joint has been an obstacle.
In accordance with the above background, various types of lead-free solders that do not contain lead have appeared. Among such lead-free solders, high-melting point solder such as AuGe is attracting attention as a bonding material for wide-gap semiconductor devices. However, the high price of the material and the rise in mounting temperature causes the process cost to rise, thus, hindering its popularization. Further, since most lead-free solders exhibit lower wettability than lead solder, the solder does not spread on the die pad part, causing parts of the die such as the corner to lack solder. Thus, the risk of bonding defects occurring at such corner is a problem. Since the wettability of lead-free solder tends to deteriorate with the rise in melting point, it is difficult to achieve both heat resistance and mounting reliability.
In order to solve the above problem, studies on diffusive sintering-type solder, such as Cu/Sn-based solder, is underway. Although such diffusive sintering-type solders show low melting point at mounting, since the melting point increases irreversibly after the diffusive sintering reaction, it is more advantageous than conventional lead-free solders in achieving both heat resistance and mounting reliability. However, even in diffusive sintering-type solders, because the wettability is slightly inferior than lead solder, for the bonding of large-area devices such as those of 5 mm×5 mm or more, there is still the risk of lacking solder. Further, since the sintered body is hard and brittle, and exhibit low stress relaxation property, which causes low heat-resisting fatigue property, there is the disadvantage of shorter device life.
Other than the above-described lead-free solder, Ag pastes are being used, but the drastic increase in material cost and contamination by the migration of Ag ion are seen as problems.
Further, in general, many lead solders and lead-free solders often contain flux such as carboxylic acid and alcohol for the purpose of removing metal oxide films. However, it has been known that such flux components tend to absorb moisture and bleed out, and the absorbed moisture and the flux that bleed out both have adverse effects on the anti-reflow reliability after moisture absorption (MSL) in the bonded package of semiconductor devices. Thus, conventionally, flux rinsing was said to be necessary after soldering, but such labor and the processing of waste fluid is being viewed as problematic. Nevertheless, if the carboxylic acid and/or alcohol is not added, in order to decrease moisture-absorption and bleed-out, the oxide film removability becomes insufficient, causing the appearance of electric conductivity and other properties to be difficult.
Further, in Patent Document 4, in the adhesive composition that utilizes silver as an inorganic filer, which is used in semiconductor devices, the use of thiols and thioethers is disclosed. However, silver is highly expensive and is likely to undergo ion migration, and thus there is a large risk of unintentional short-circuiting between electrodes.